Surface Plasmon Resonance (SPR) is a physical phenomenon that is commonly used to investigate the binding properties of chemical and biological molecules. Analytes may bind to immobilized probe molecules on a metal film, altering a resonance characteristic of the surface plasmon and changing the refractive index around the metal film. SPR systems can detect such changes in refractive index. These changes can be measured as an angle shift or alternatively, as a wavelength shift.
FIG. 1 shows a SPR measurement system. A light source 11 projects a beam of light onto a target 12. The light is reflected off the target onto a photo detector 13.
Some SPR systems use incoherent sources such as Light Emitting Diodes (LEDs). The LED can give adequate performance under some conditions, but its broad spectrum of incoherent light poses a Signal to Noise Ratio (SNR) problem that makes it unsuitable when high resolution and accuracy are needed.
Resolution and accuracy of measurement results can be improved moderately by increasing the power of an incoherent light source in a SPR system. As optical power increases, SNR of the measured signal at photo detector 13 improves. Unfortunately, this approach does not fully solve the problem because LEDs provide limited power density due to their inherent design.
To overcome this limitation, a laser can be used in place of a LED. A laser can provide the optical power density necessary to facilitate tests with better resolution. A laser under normal operating conditions, i.e. when driven well above the threshold, produces high power and coherent light. But under these normal operating conditions, high power coherent light sources introduce optical interference that leads to distorted measurements because of random shifts in standing wave patterns. These shifts in the standing wave patterns degrade the ability of an SPR measurement system to resolve minute changes in the refractive index and, therefore, limit the ability of the system to measure binding analytes.
There remains a need for a way to improve the resolution and accuracy of SPR systems.